The most common display mode in use today in display systems such as personal computers or display adapters for such computers can be referred to as an "indirect color mode". In this mode of operation pixel data, contained in a display memory, are used indirectly to define the chrominance and/or luminance values of the pixels for display.
In a display system having an all-points addressable (APA) display memory, a number, typically between 1 and 8. of bits is stored for each of the picture elements (or pixels) of a display field (corresponding. for example to the screen of a display device). When operating in an indirect color mode, these bits in the display memory form an index for accessing a location within a look-up table called a palette. The addressable locations in the palette contain display data defining the chrominance and/or luminance for a pixel having an index addressing that palette location. In the case of a display system for controlling a color cathode ray tube, or another display device where colors are generated by mixing the primary colors red, green and blue, the palette entry will contain chrominance and/or luminance data determining an intensity for each of those colors. If the display device requires analogue signals for controlling the physical display processes, the chrominance and/or luminance data output from the palette are then converted into analogue signals in digital-to-analogue converters (DACs) for controlling the display device (eg. in the case of a color CRT. the color guns).
In an indirect color mode it is usual for up to eight bits to be stored for each pixel in a display memory and for these to be translated in a look-up table, or palette into three values which define the intensity of the red, green and blue elements of the pixel. The standard indirect color mode which has been adopted in many existing display adapters, such as those for the IBM PS/2 (trademark of International Business Machines Corp.) computer family, is for the palette to generate 6 bits each for red, green and blue, making 18 bits in all. Where the display memory comprises 8 bit planes (ie. 8 bits per pixel) it allows the simultaneous display of up to 256 (2**8) different colors, chosen from the 256K (2**18) possible different colors.
For some applications, however, it is not sufficient to be able only to specify 256 colors simultaneously. In this case, a greater number of bits per pixel are needed. If 16 bits are used. 64K (2**16) different colors can be specified at a time. The possibility of specifying one of 64K different colors makes a palette unnecessary. Also, using 16 bits to address a palette would make it uneconomically large. Accordingly, if the display memory can be arranged to define a sufficiently large number of bits per pixel to enable all the desirable colors to be specified directly, it is possible to operate in a so-called "direct color mode" where the values stored in the display memory control the display device directly via the DACs.
If it is desired to enable a display system to operate in both an indirect mode and a direct mode. a problem arises in that different numbers of bits for controlling the digital-to-analogue converter stage may be generated in the indirect and direct color modes. Consider the case as described above where 18 bits of chrominance and/or luminance data are generated for each pixel in an indirect color mode. In order to accommodate this, the digital-to-analogue converter stage will, in the indirect color mode, have to convert the 18 chrominance and/or luminance data bits from the palette into analogue chrominance and/or luminance values for controlling the display of the pixels. If, however. 16 chrominance and/or luminance data bits (ie. 2.times.8 bits) in the display memory are used in a direct color mode for defining directly the color and/or luminance of the pixels there will be two inputs to the converter stage which will not be specified.
It will be appreciated that the numbers mentioned here are purely illustrative of the general problem of how to provide a display system which is able to operate in both a direct and an indirect color mode, where a different number of bits for defining the chrominance and/or luminance of the pixels in the different modes.